1. Field of the Invention
The present invention relates to a wavelenth-changeable or tunable light source using a laser, and particularly to a wavelength-changeable or tunable light source using a semiconductor laser (hereinafter also referred to as LD), an optical communication network using the light source and a wavelength control method for controlling the wavelength of output light from the light source.
2. Related Background Art
Study and development of a wavelength-changeable light source have been increasingly advanced as an important key device in the fields of wavelength division multiplexing communications, optical measurements and so forth. For example, study and development of a single-wavelength operative LD, such as a distributed feedback laser diode (DFB-LD) and a distributed Bragg reflector laser diode (DBR-LD), have been promoted. An example thereof will hereinafter be described.
FIG. 7 is a block diagram illustrating a wavelength-changeable light source using a two-electrode DFB-LD. The light source includes a two-electrode DFB-LD module 701, a two-output current source 702, a temperature controller 203 and a wavelength control system #4 (703).
The two-electrode DFB-LD module 701 is a device in which its current-injection electrode is divided into two portions and the wavelength of its light output can be changed by controlling a current injected into the device. An example thereof is disclosed in "Journal of Electronics Letters, volume 22, No. 22, pp. 1153-1154". In this example, the lasing wavelength is in a range of 1556 nm to 1558 nm and thus a wavelength-changeable range of about 2 nm is attained. Further, some manufacturers presently sell such a device as a module for the use of study. The two-electrode DFB-LD module 701 is constructed by packaging the above two-electrode DFB-LD together with an optical coupling system, an optical isolator, an optical fiber, a Peltier element, a thermistor and so forth. Since the lasing wavelength of the two-electrode DFB-LD shifts due to a change in its ambient or environmental temperature, the device temperature of the two-electrode DFB-LD is controlled by the Peltier element and the thermistor and thus a change in the lasing wavelength due to the temperature change is controlled. The optical isolator prevents the return of light into the two-electrode DFB-LD, and hence stabilizes the lasing wavelength of the two-electrode DFB-LD.
Further, the two-output current source 702 is an electric current source which has two independent outputs. The output currents of the current source 702 are set by a current control signal input from its outside (i.e., from the wavelength control system #4). The temperature controller 203 causes a current to flow into the thermistor (which is arranged in the two-electrode DFB-LD module) and measures the temperature by detecting a voltage between the thermistor terminals. The temperature controller 203 further drives the Peltier element (which is also arranged in the two-electrode DFB-LD module) having heat-generation and heat-absorption characteristics due to a current injected thereinto such that the measured temperature reaches a target temperature. The Peltier element can increase or decrease the temperature of a heat sink on which the two-electrode DFB-LD is mounted. The target temperature can be set in the temperature controller or by using a temperature control signal from its outside. Further, a difference between the target temperature and the measured temperature is output as a temperature control monitor signal. In this example, the target temperature is internally set. The wavelength control system #4 (703) controls the two-output electric current source 702 by using the current control signal, and controls the wavelength of the two-electrode DFB-LD module 701.
FIG. 8 illustrates another example of the wavelength-changeable light source using a DFB-LD. The light source is comprised of a single-electrode DFB-LD module 201, a current source 202, a temperature controller 203 and a wavelength control system #5 (801).
The single-electrode DFB-LD module 201 is a device that is presently sold commercially as a module by several manufacturers. Since the device only has a single electrode, its lasing wavelength can not be largely varied by a current injected thereinto. The ratio of a change in the lasing wavelength relative to the injected current is small, such as about 0.008 nm/mA, and the light output is also varied as the injected current increases. Therefore, the wavelenth-changeable range due to the current is in the order of 0.1 nm. For this reason, the wavelength is changed by using a change in the temperature in this example. For instance, the ratio of a change in the wavelength relative to the temperature is about 0.08 nm/.degree. C. and thus the wavelength-changeable range in the order of nanometer can be obtained.
The DFB-LD module 201 is constructed by packaging the above DFB-LD together with the optical coupling system, the optical isolator, the optical fiber, the Peltier element, the thermistor and so forth. For example, in a DFB laser diode manufactured by Fujitsu Limited, FLD150F2KP (a trade name), a threshold current is 20 mA, a forward voltage 1.1 V (IF=30 mA), a standard value of its peak lasing wavelength is 1550 nm and a maximum of its spectral half width is 0.2 nm. This is a light emitting device with a single mode fiber. The inventor of the present invention measured characteristics of that light emitting element, and obtained the characteristics shown in FIGS. 9A and 9B. FIG. 9A shows the characteristic of the lasing wavelength relative to the temperature, and FIG. 9B shows the characteristic of the lasing wavelength relative to a supplied current. It can be known from those measurement results that the wavelength can be varied in a range having a width of 2 nm by the temperature control between 15.degree. C. and 35.degree. C. and that the wavelength can be varied in a range having a width of 0.35 nm by the current control between 30 mA and 70 mA.
The current source 202 is a single-output current source. Its output current can be controlled by the internal setting or by the current control signal input thereinto from outside. In this example, the internal setting is performed. The temperature controller 203 is the same as illustrated in FIG. 7. In this example, the voltage between the thermistor terminals from the DFB-LD module 201 is detected by the temperature controller 203, and the temperature control monitor signal is recognized by the wavelength control system #5. In addition thereto, the wavelength control system #5 outputs the temperature control signal, by which the DFB-LD module 201 is set to a desired wavelength, on the basis of that temperature control monitor signal. Accordingly, the temperature setting is performed by controlling the temperature controller 203 using the temperature control signal from outside (i.e., from the wavelength control system #5). The wavelength control system #5 (801) thus controls the temperature controller 203 by using the temperature control signal, and controls the lasing wavelength of the DFB-LD module 201. On the other hand, the wavelength control system #5 (801) monitors the condition of the temperature control by using the temperature control monitor signal from the DFB-LD module 201.
The above-discussed wavelength-changeable light sources, however, have the following disadvantages.
The drawback of the example using the two-electrode DFB-LD will be initially described. This device has been only produced on trial, and its fabrication process for mass production has not yet been established and hence its cost is high. Situations of other multi-electrode wavelenth-changeable LDs, such as three-electrode DFB-LDs and three-electrode DBR-LDs, are the same. Therefore, though those device have the wavelength-changeable range having a width of 2 nm, the supply of those devices having sufficiently stable characteristics is not yet achieved.
The drawback of the example using the temperature control will next be described. Generally, the response of a temperature control system is slow. The same is also true in the temperature control system of the LD module in which the temperature is detected by the thermistor and the temperature is controlled by the Peltier element. Specifically, it is difficult to settle its control within one second. Further, as the settling time of the control is shortened, overshooting is likely to occur. When such a device is used as a light source in wavelength division multiplexing communications with narrow intervals between channels, crosstalk is likely to occur during the time of changing the wavelength.
It is an object of the present invention to provide a wavelength-changeable or tunable light source in which a current control with a speedy response and a narrow wavelength-changeable range is combined with a temperature control with a slow response and a wide wavelength-changeable range, hence the wavelenth-changeable range having a width in the order of a nanometer is achieved even when a single-electrode DFB-LD is used and time required for the wavelength changing operation is shortened.